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Natural gas: transition fuel or greenhouse menace?

Say what you will about coal, but at least it stays where it’s put. On its way to the user, coal doesn’t gush from the rail trucks, spreading itself through the atmosphere and warming it at about 70 times the rate of carbon dioxide.

Natural gas is different. A new draft study provides evidence that, in the US, enough natural gas leaks into the air to give gas-fired electricity, megawatt-hour for megawatt-hour, a bigger greenhouse impact than electricity from good-quality steaming coal.

This news will be unnerving for the many people who point to natural gas as a relatively clean alternative to other fossil fuels. Conventional wisdom has been that a state-of-the-art natural gas power plant can produce electricity with barely 30% of the carbon dioxide emissions of a typical brown coal plant, and less than half of those for black coal.

The modest climate impact of natural gas, however, applies only from the point where the gas is burnt. In the study mentioned above, Professor Robert Howarth of Cornell University in the US examines the fuel’s broader effects. “The most recent data I could find for the US (from 2006)”, Howarth reports, “suggest a leakage rate from the oil and gas industry of an amount of methane equal to 1.5% of the natural gas consumed.”

Methane, with the chemical formula CH4, makes up about 87% of natural gas after various contaminants have been removed.

Howarth’s figure of 1.5% is consistent with data from various sources including the US Environmental Protection Agency. He acknowledges that his estimate has a large margin for error, since leaks in the gas industry are not well monitored. But 1.5%, he contends, is a conservative figure, and US government scientists and gas industry officials quoted in the New York Times on October 14 last year agree that the real amount is almost certainly higher.

If Howarth is anywhere near correct, suggestions by some environmentalists that natural gas can act as a “bridge” for a low-carbon transition from coal-fired power to renewable power fall by the wayside.

Howarth’s premises are not uncontroversial. To calculate the warming impact of methane, he uses the figure of 72 times the global warming potential, per volume, of carbon dioxide. This is the impact over the first 20 years after the methane reaches the atmosphere. The figure usually cited for the warming potential of methane is 25 times that of carbon dioxide, measured over 100 years.

Howarth’s choice here is the correct one. The figure of 72 times is appropriate to the key danger which natural gas leaks pose to the environment: their ability, due to their short-term greenhouse potency, to help trigger quick-acting “positive climate feedbacks”.

In the atmosphere, methane reacts with oxygen to form water and carbon dioxide. After seven years, half of the methane is gone, and after 20 years little remains. But a big pulse of the gas, its effects amplified by quick-acting feedbacks, could within a few years raise temperatures to the point where they exceeded natural “tipping points”. The climate would then “flip” to a new, hotter state.

One such positive feedback is seen in the impact of greenhouse gases on Arctic permafrost. Warming in the Arctic causes permafrost to melt, and in the oxygen-poor conditions of the resulting swamps, bacteria turn organic matter into methane — which warms the environment still further.

Methane in the atmosphere is now at about two-and-a-half times the level in 1750. It now accounts for about 20% of the warming effect of all long-lived greenhouse gases. From 1998, concentrations of methane stopped rising for a period, perhaps due to the drying of tropical wetlands.

The last two years, however, have seen levels trend upward once again. As well as reflecting the amounts of methane now bubbling from Arctic swamps, this renewed rise may stem from a boom in the extraction — and leaking — of natural gas.

Over the next 20 years, a big expansion of natural gas production could help bring about the methane pulse that humanity should be dreading.

At present, the April 24 Weekend Australian reported, natural gas furnishes about 24% of the world’s primary energy supply. By 2030, the paper quotes the International Energy Agency as predicting, global demand for natural gas will rise by more than 67%.

In Australia, the Australian Bureau of Agricultural and Resource Economics (ABARE), predicts natural gas production will quadruple in the next 20 years as the country becomes a major liquefied natural gas exporter.

ABARE puts Australia’s 2030 export revenues from liquefied natural gas at about $50 billion in current dollars — almost twice the current annual income from iron ore exports.

Most of the extra natural gas entering world markets will be from conventional fields. Growing amounts, however, will be from unconventional sources that include gas shales and coal seams. Gas-bearing shales underlie huge areas of the US, and new drilling technology is now making them economic to exploit. The March 11 Economist predicts North America enjoying “an unforeseen surfeit of natural gas”, with US reserves set to last more than a century.

In Australia, the Weekend Australian said, close to 600 wells are now being drilled each year to extract coal seam gas, principally in Queensland. By 2029-2030, ABARE estimates, coal seam gas will provide close to 90 per cent of the gas consumed in eastern Australia.

With current practices in the gas industry, and the leakage of methane that results, using gas to meet growing world energy demand would be roughly the same as letting the coal industry continue and expand — that is, environmental suicide.

But what if the industry cleans up its act? Gas that leaks is gas that cannot be sold, or used by purchasers to turn a profit. All along the supply chain, there are financial incentives to stop gas escaping.

New technology, based on radar and infrared photography, is now being used to detect leaks. But the chances of making big cuts in the amount of natural gas that reaches the atmosphere appear dim.

Much of the new demand will be from China and India, where gas infrastructure is often poorly maintained. Globally, discoveries of large new conventional gas fields have become rare, and new supply will come increasingly from gas shales and coal seams.

These sources are highly dispersed, involving large numbers of wells connected by small pipelines. In Queensland alone, according to the Weekend Australian, some 30,000 wells are projected. Drilling operations routinely allow gas to be vented, and even when the wells are finished and connected, the complexity and spread of the infrastructure means that accidental releases will be numerous.

The only real solution is to bypass natural gas, along with other fossil fuels. Like coal, gas must be phased out. Government policy should focus on a rapid development of the renewable energy technologies that are available now, or on the cusp of commercialisation.

First, the article argues that the global warming potential of methane is 72 (i.e., 72 times as potent as carbon dioxide) on a volumetric basis over 20 years. The internationally-accepted value (i.e, the value accepted by the Intergovernmental Panel on Climate Change) is about 25 on a mass basis over 100 years and higher (I will accept the value of 72 here) for a 20-year period. On a volumetric basis, the IPCC value of 25 would be about 8, which is only one-ninth the value used in the article. The article argues that the 20-year time frame is more appropriate, but that is based on the assumption that a tipping point will be reached soon. The assumption of a tipping point in a couple decades is arguable.

To avoid unnecessary argument with you, I will use the value of 72 on a mass basis, not on a volume basis.

Consider emissions from coal-fired electricity.

Carbon dioxide emissions from burning the coal are typically in the range of 0.9 to 1.0 tonnes CO2 (tCO2) per MWh.

Coal mining causes the release of methane that is embedded in the coal. The amount varies from very small to about 10 cubic meters per tonne of coal in "gassy" mines. Using a mid-range figure of 5 cubic meters per tonne of coal. One tonne of coal produces about 3 MWh of electricity, so we have 1.67 cubic meters of methane per MWh. At a methane density of 0.7 kg/m3, we have 1.2 kg CH4/MWh, which yields a CO2 equivalent of 0.084 tCO2equiv/MWh.

Now, consider electricity generation from natural gas in a combined cycle plant with a heat rate of 7.5 MMBtu/MWh (MMBtu = million British thermal units).

It takes about 215 cubic meters of methane to yield the 7.5 MMBtu needed to generate one MWh of electricity. The article you cite gives a methane leakage rate of 1.5%, which would be 3.2 cubic meters (about twice the per MWh emission of methane for coal in my mid-range estimate above. The 1.5% leakage rate for natural gas yields about the same per MWh emission rate as coal if we consider a highly gassy coal mine instead of the mid-range coal mine).

The leakage of 3.2 cubic meters of methane per MWh is equivalent to 2.2 kg, which at a global warming potential of 72, yields 0.16 tCO2equiv/MWh.

In summary, we have

For Coal: 0.9 to 1.0 tCO2/MWh from coal combustion plus 0.08 tCO2equiv from methane leakage from coal mines; for a mid-range total of 1.03 tCO2equiv/MWh.

For Natural Gas: 0.4 tCO2/MWh from gas combustion plus 0.16 tCO2equiv from gas leakage; yielding a total of 0.56 tCO2equiv/MWh.

Therefore, natural gas combined cycle emits 54% as much as coal-fired steam cycle. The ratio would be less than 50% if we used the more conventional global warming potential for methane of 21, instead of 72.

The article you cite notes that the 1.5% leakage rate for natural gas may be on the low side, but it provides no hint as to what a more realistic value would be, so I think it is best for now to stick with the 1.5% value.

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